Apparatus and method for controlling etch depth

ABSTRACT

An apparatus and method for etching a feature in a wafer with improved depth control and reproducibility is described. The feature is etched at a first etching rate and then at a second etching rate, which is slower than the first etching rate. An optical end point device is used to determine the etching depth and etching is stopped so that the feature has the desired depth. Two different etching rates provides high throughput with good depth control and reproducibility. The apparatus includes an etching tool in which a chuck holds the wafer to be etched. An optical end point device is positioned to measure the feature etch depth. An electronic controller communicates with the optical end point device and the etching tool to control the tool to reduce the etch rate part way through etching the feature and to stop the etching tool, so that that the feature is etched to the desired depth.

FIELD OF THE INVENTION

[0001] The present invention relates generally to apparatus and methodsfor etching features as part of semiconductor device fabricationprocesses, and more particularly to an optically controlled method andapparatus allowing accurate control of the depth of a feature beingetched.

BACKGROUND OF THE INVENTION

[0002] There are a number of competing pressures in the fabrication ofsemiconductor devices. It is important that devices are fabricatedaccurately so as to either avoid device failure or to reduce the numberof devices on a wafer that fail. It is also important that devices meetmanufacturing specifications to ensure that the devices operatecorrectly even if they do not fail. There is also a requirement ofreproducibility so that devices fabricated from different wafers allmeet manufacturing specifications. Even if an acceptable level ofreproducibility is in fact met, there can also be a perceivedreproducibility in which designers of devices require comfort thatmanufacturing specifications can always be met and are not being metmerely as a matter of luck.

[0003] There is also the competing pressure of throughput which requiressemiconductor device manufacturing tools to operate quickly in order toprovide a required throughput of devices. However, speed of fabricationtends to militate against the reproducibility and accuracy of devicefabrication.

[0004] A common step in the fabrication of semiconductor devices isetching a feature into a layer of a wafer. The depth of the feature canoften be a critical factor in the correct operation or failure of thedevice or otherwise a key manufacturing specification.

[0005] One mechanism by which people have tried to ensure that featureswith the correct depth are etched is to provide an etch stop layer inthe wafer prior to etching. The presence of an etch stop layer providesa way to prevent the feature from being etched deeper than the etch stoplayer, but requires a more complicated wafer structure to start with andis therefore complex and costly. Also, in some devices an etch stoplayer cannot be used as it would interfere with the correct operation ofthe device. Further, it can be necessary to etch into a wafer substratewhich does not include an etch stop layer.

[0006] Various optical techniques have also been used to control etchprocesses. For example in a gate manufacturing process, optical emissionspectroscopy can be used to determine when a layer of polycrystallinesilicon has been etched through. The emission spectrum changes when thegate oxide layer is exposed and begins to etch and so the gate oxidelayer can be detected. However, again this requires the presence of aspecial layer effectively acting as an etch stop indicator in the wafer.Further some etching of the gate oxide layer needs to occur in order togenerate the change in the emission spectrum and so the depth of theetch through the polysilicon layer cannot be carefully controlled.

[0007] Another method which does not provide sufficiently accurate etchdepth control is the use of interferometry based techniques. A singlestep etch is used to etch the feature and an interferometric end point(IEP) device is used to measure the relative change in depth of thefeature that has been etched into the wafer. When the desired relativechange in etch depth is measured, the etching is stopped. However, highetch rate processes cannot stop etching immediately and in areproducible manner and so there tends to be a significant variation inthe actual depth etched. The lack of control of etch depth and lack ofreproducibility can lead to device failure or to failure to meetmanufacturing specifications or to wafer-to-wafer variations that do notmeet device designers requirements.

[0008] There is therefore a need for a simple, reproducible method foraccurately controlling etch depth while still providing sufficientprocessing throughput.

SUMMARY OF THE INVENTION

[0009] A method for etching a feature to a desired depth in a wafer isdisclosed. The method includes etching the feature at a first etchingrate. The feature is then etched at a second etching rate, which isslower than the first etching rate. The etching depth is opticallydetermined and etching is stopped so that the feature has the desireddepth. Using two different etching rates provides high throughput withgood depth control. Using a second etching rate slower than the firstetching rate facilitates improved optical end point resolution.

[0010] According to another aspect, the invention provides a method foretching a trench in a silicon layer of a wafer. The method includesetching at a first etch rate and then etching at a second etch rateslower than the first etch rate. The current etch depth is opticallydetermined and etching is stopped so that the trench depth reaches adesired end point.

[0011] According to a further aspect, the invention provides apparatusfor etching a feature in a wafer. The apparatus includes an etching toolincluding a chuck for holding the wafer. An optical end point device isprovided and positioned to measure the etch depth. An electroniccontroller communicates with the optical end point device and theetching tool. The controller controls the tool to reduce the etch ratepart way through etching the feature and to stop the etching tool, sothat the feature is etched to the desired depth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention, together with further advantages thereof, may bestbe understood by reference to the following description taken inconjunction with the accompanying drawings in which:

[0013]FIG. 1 is a schematic cross sectional view of etching apparatusaccording to an aspect of the invention;

[0014]FIG. 2 is a schematic cross sectional view of a wafer having atrench etched in it according and illustrating the etching method of theinvention;

[0015]FIG. 3 shows a flow chart illustrating the etching method of theinvention; and

[0016]FIG. 4 is a schematic cross sectional view of another embodimentof etching apparatus according to an aspect of the invention.

[0017] In the Figures, like reference numerals refer to like componentsand elements

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to methods and apparatus forcontrolling the depth of a feature etched in a wafer during fabricationof a semiconductor device.

[0019]FIG. 1 is a schematic view of a plasma processing system 100,including a plasma processing tool 101. The plasma processing tool 101is an inductively coupled plasma etching tool and includes a plasmareactor 102 having a plasma processing chamber 104 therein. Atransformer coupled power (TCP) controller 150 and a bias powercontroller 155 respectively control a TCP power supply 151 and a biaspower supply 156 influencing the plasma 124 created within plasmachamber 104.

[0020] The TCP power controller 150 sets a set point for TCP powersupply 151 configured to supply a radio frequency signal at 13.56 MHz,tuned by a TCP match network 152, to a TCP coil 153 located near theplasma chamber 104. An RF transparent window 154 is provided to separateTCP coil 153 from plasma chamber 104 while allowing energy to pass fromTCP coil 153 to plasma chamber 102. An optically transparent window 165is provided by a circular piece of sapphire having a diameter ofapproximately 2.5 cm (1inch) located in an aperture in the RFtransparent window154.

[0021] The bias power controller 155 sets a set point for bias powersupply 156 configured to supply an RF signal, tuned by bias matchnetwork 157, to a chuck electrode 108 located within the plasma chamber104 creating a direct current (DC) bias above electrode 108 which isadapted to receive a substrate 106, such as a semi-conductor waferworkpiece, being processed A gas supply mechanism or gas source 110includes a source or sources of etchant gas or gases 116 attached via agas manifold 117 to supply the proper chemistry required for the etchingprocess to the interior of the plasma chamber 104. A gas exhaustmechanism 118 includes a pressure control valve 119 and exhaust pump 120and removes particles from within the plasma chamber 104 and maintains aparticular pressure within plasma chamber 104.

[0022] A temperature controller 180 controls the temperature of heaters182 provided within the chuck 108 by controlling a heater power supply184.

[0023] In general terms, in plasma chamber 104, substrate etching isachieved by exposing substrate 106 to inonized gas compounds (plasma)under vacuum. The etching process starts when the gases are conveyedinto plasma chamber 104. RF power delivered by TCP coil 153 and tuned byTCP matching network 110 ionizes the gases. The power delivered byelectrode 108 induces a DC bias on substrate 106 to control thedirection and energy of ion bombardment of substrate 106. During theetching process, the plasma reacts chemically with the surface of thesubstrate 106 to remove material not covered by a photoresistive mask.

[0024] In one preferred embodiment of the invention, a suitable plasmaprocessing tool would be the 2300 Versys Silicon Etch System, asprovided by Lam Research Corporation of Fremont, California.

[0025] In one embodiment, the etching system includes a singlewavelength interferometric end point device 160 located external to theplasma processing chamber 104. Optical access to the plasma processingchamber is provided by window 165 comprising a sapphire insert,approximately 2.5 cm (one inch) in diameter, in an aperture in the RFtransparent window 154. The end point device 160 is positioned adjacentto the window 165 and is positioned so as to be able to measure thedepth of features etched into the wafer 106 in a direction substantiallyperpendicular to the plane of the wafer. The end point device 160generates and transmits a substantially single wavelength, or narrowband (bandwidth ≦10 nm), of light which is reflected from the wafersurface and monitored in real time during an etch process as will bedescribed in greater detail below. In one embodiment of the invention,the end point device 160 is provided as an integrated part of the plasmaprocessing chamber 104 or etching tool 101.

[0026] The etching system 100 also includes electronic control circuitry170 in communication with the end point device 160 and the etching tool101. The electronic control circuitry includes electrical and opticaldevices to process the optical signals from the end point device toprovide electrical signals indicating the current depth of an etchedfeature and also electrical signals to control the operation of theetching tool. The electronic control circuitry 170 can be in the form ofa suitably programmed general purpose digital computer. The electroniccontrol circuitry 170 constantly monitors the relative change in depthof a feature being etched in the wafer and can control the etchingoperation of the etching tool according to the etching method describedbelow.

[0027] In another embodiment of the invention, a broadband (spectralrange approximately 190-1000 nm) reflectometry based technique anddevice are used to optically determine the end point of an etch process,instead of the single narrow wavelength band technique and devicereferred to above. The broadband measuring device and technique canprovide an absolute measure of feature depths. The technique involvesusing a deterministic approach to parametrically estimate feature depthsby matching modeled and measured broadband spectra at any instant intime. A suitable broadband reflectometry technique is described in U.S.Provisional Patent Application Serial No. 60/403,213 filed on Aug. 13,2002, entitled “Endpoint Strategies for in situ Control of Recess andDeep Trench Etch Processes” in the names of Vijaykumar C Venugopal andAndrew J Perry and U.S. Provisional Patent Application Serial No.60/408,619 filed on Sep. 6, 2002, entitled “Reflectometry-basedApproaches For in situ Monitoring of Etch Depths in Plasma EtchingProcesses” in the names of Vijaykumar C Venugopal and Andrew J Perry,both of which are hereby incorporated by reference in there entirety forall purposes.

[0028]FIG. 4 shows a schematic cross sectional drawing of an etchingsystem 400 similar to that shown in FIG. 1, but including the broadbandend point measuring device 460. Broadband end point measuring device 460includes a source 461 of broadband radiation, connected by a length ofUV grade optical fiber 462 to a collimator 464 adjacent the sapphirewindow 465. The collimator 464 is connected by another length of UVgrade optical fiber 466 to a 190-1000 nm spectrograph 468 which isconnected to control circuitry 470. Control circuitry 470 is adapted toprocess signals from the broadband end point 460 device and determinethe etch depth in this embodiment.

[0029] In use, the etching tool 101 is controlled to strike and sustaina plasma 124 in the plasma chamber 104 which is used to etch the desiredfeature in the wafer 106. The etching method will now be described withparticular reference to FIGS. 2 and 3. FIG. 2 shows a schematic crosssection of a part of a wafer 200 etched according to the method and FIG.3 shows a flow chart 300 illustrating the etching method. An embodimentof the method will be described with reference to etching a trench in asilicon substrate layer of a wafer as part of a shallow trench isolation(STI) process.

[0030]FIG. 2 shows the wafer 200 after the etching process has beencarried out. The wafer includes a crystalline silicon substrate layer202, a pad oxide layer 204 and a silicon nitride hard mask layer 206which has previously been patterned to define the location of the trenchfeature 210 to be etched.

[0031] At the start of the method 302 a first high etch rate etch of thetrench is carried out 304. A high etch rate can be considered to be anetch rate greater than approximately 4000 Å/min. An etch rate ofapproximately 5000 to 8000 Å/min can be used for the first fast etch304. The depth of the feature being etched is monitored 306 to determinewhether the feature has reached a first depth 212 a substantial waytoward the target end point depth 214 desired for the feature. The firstdepth 212 can be more than approximately 65% of the end point depth 214,preferably more than approximately 70% of the end depth and morepreferably more than approximately 80% of the end depth. The first depth212 can be in the range of approximately 65 to 85% of the end pointdepth 214, and more can be in the range of approximately 80 to 85% ofthe end point depth 214.

[0032] The high etch rate etch can be carried out using the followingoperating conditions and recipe: a plasma pressure in the range ofapproximately 10 to 70 mT, a TCP power in the range of approximately 500to 1400 W, a bottom electrode bias in the range of approximately 0 to800 W and an etchant gas composition including Ar, C1 ₂, HBr, CF₄, O₂,SF₆ and He. Any suitable etchant gas mixture and etching tool operatingparameters can be used which provides the required high etch rate etchof the feature in the wafer 200.

[0033] The progress of the etch is monitored 306 in situ by the opticalend point device 160 and control circuitry 170 and the first high rateetch 304 is continued 307, until it is determined 306 that the currenttrench depth has reached the first depth 212. The high etch rate processprovides a good profile to the bottom of the trench 216 including asmooth rounded bottom surface. This is advantageous as it helps to avoidthe formation of voids when the trench is filled with an oxide materialand so helps to obviate device failure.

[0034] A second, slower etch is then carried out 308 which has a loweretch rate than the first etch. An etch rate of less than approximately3000 Å/min can be considered slow for an STI process. However, the rateof the second etch step is selected so that it substantially preservesthe smooth and rounded profile of the trench bottom so that thecompleted trench has the desired profile.

[0035] When the optical end point device and control circuitry determinethat the cross over depth 212 has been reached, the etching tool iscontrolled to reduce the etching rate so that the trench is etched tothe desired end point depth 214 at a slower rate. This can be achievedby changing the composition of the etchant gas such that HBr is used asthe source of etchant species and changing the operating parameters ofthe etching tool, although other methods can be used to provide thedesired slow etching rate. The following etching gas composition andetching tool operating parameters can be used to provide an etch rate ofapproximately 1000-3000 Å/min: a plasma pressure in the range ofapproximately 10 to 80 mT, a TCP power in the range of approximately 200to 1200 W, a bottom electrode bias in the range of approximately 0 to500 W and an etchant gas composition including Ar, C1 ₂, HBr, CF₄, O₂,SF₆ and He.

[0036] The progress of the slower etch rate etch is monitored 310 insitu by the optical end point device 160 and control circuitry 170 todetermine whether to stop the etch so that the trench will land at thedesired end point depth 214, or keep etching 311. Carrying out a slowertrench depth landing 308 means that greater resolution can be providedby the optical end point measuring device and more time is available inwhich to control the etching tool so as to stop the etch at anappropriate time so that the trench depth is correct. So the accuracy ofmeasurement of the depth of the trench is improved. When it isdetermined 310 that the end point has been reached, the etching tool iscontrolled to stop etching 312 the trench. The etching process then ends314 and provides an etched trench with the desired depth 214 and alsohaving the desired profile and with a high throughput rate, as the bulkof the etching has been carried out at a high etch rate.

[0037] Etching can be stopped 312 either when the measured depthcorresponds to the desired feature depth 214, or alternatively, beforethe measured depth reaches the desired feature depth, for example ifthere is some ‘overshoot’ in the etching process. The latter embodimentcan be used where it is not possible to instantaneously stop etching. Inthat case, this is compensated for by controlling the etching tool tostop etching before the desired depth is actually reached so that thetrench actually lands at the desired depth rather than overshooting thedesired depth.

[0038] When using high etch rates only, the variations in end pointtriggering time, plasma ignition, RF ramping and match tuning from waferto wafer can be sufficient to put the wafers outside of the acceptablerange of reproducibility. Indeed variations in less than 1 second of theprocessing time can be sufficient for trenches in wafers from differentprocesses not to meet manufacturing reproducibility tolerances. Byadding the slower etch rate step, the invention not only improves theresolution and therefore accuracy with which the end point can bedetermined, but also improves the reproducibility of the process fromwafer to wafer as any variations in the total process time caused bylags or variations in the etching tool will not create as large avariation in the trench depth owing to the slow etching rate up to theend point. Further, a slow etch rate may not provide the desired trenchprofile properties. However, using a high etch rate process, allows thedesired trench profile to be generated, which can then be usefullypropagated by the slow etch, provided that the slow etch is not carriedout for a length of time sufficient to significantly alter the trenchprofile. Also the combination of high and low etch rate means that ahigh manufacturing throughput rate of wafers with high reproducibilitycan be provided.

[0039] When using a single narrow wavelength band IEP device andtechnique, the depth of the trench is monitored constantly as thistechnique can only measure the depth relative to a starting position(e.g. the wafer surface prior to starting the etch). If a broadbandreflectometry technique, or other optical end point technique which canmeasure absolute depth values, is used, then the depth of the trenchdoes not need to be constantly monitored throughout the entire etchprocess, but only toward the end, when it is necessary to determine howmuch further to etch using the slow rate, or additionally, to determinewhen to stop the fast etch and change over to the slow etch. In oneembodiment of the invention, the method can include using the currentmeasured depth of the trench to determine when to change to the sloweretch rate and can control the etching tool to automatically change theetch rate. Alternatively, the first high rate etch can be carried outfor a fixed period of time after which the etch rate is decreased andthe depth of the trench is measured to ensure landing at the desireddepth.

[0040] The method is not limited to the use of two different etch ratesonly. For example, three or more different etch rates could be used witha corresponding number of cross over depths. Further, the cross overbetween etching rates need not be instantaneous (step change) and can bea gradual (continuous) change in the etch rate, so that the etch ratechange occurs over a region rather than at a specific depth.

[0041] The above description has been in the context of an STI process,but the invention is not limited to such processes. For example, theinvention can also be used in recess processes. A recess process istypically used in the fabrication of memory cell devices. A trench isetched in a layer of silicon and a collar of dielectric material isfabricated around the top of the trench. The trench is filled with anamount of polysilicon which overlaps an amount of the dielectric collarso as to provide a capacitive device having a desired capacitance.However, variations in the trench depth would change the amount ofpolysilicon overlapping the dielectric collar and therefore thecapacitance. Therefore careful silicon trench depth control is animportant aspect of fabricating such devices.

[0042] What can be considered ‘high’ and ‘low’, or ‘fast’ and ‘slow’,etch rates will depend on the context of the depth of the feature beingetched. If a 5% variation is depth is an acceptable reproducibilitymetric, then 5% of a very deep feature is a greater distance than 5% ofa very shallow feature, and so can be etched correspondingly morequickly. For example in the STI process described above a typical trenchdepth can be in the range of approximately 2,000 to 5,000 Å, in whichcase a high etch rate can be approximately 4000 Å/min and higher and alow etch rate can be 3000 Å/min and lower. For deep trench processestrench depths of 100,000 to 150,000 Åcan be required. In this context,high etch rates can be 10,000 Å/min and higher and low etch rates can be5,000 Å/min and lower.

[0043] The ratio of etch rates (high:low) can be greater than 1.3:1,preferably greater than 1.5:1, more preferably greater than 2.5:1 andmost preferably greater than 3.5:1, The etch rate ratio (high:low) canbe in the range from about 1.5:1 to 2.5:1, preferably from about 2.5:1to 3.5:1 and more preferably about 3.5:1 to 10:1.

[0044] Therefore the present invention can be used in the etching ofvarious features, and not just trenches, and in various fabricationprocesses to provide improved etch depth control and wafer to waferreproducibility.

[0045] Although the foregoing invention has been described in somedetail for purposes of clarity of understanding, it will be apparentthat certain changes and modifications may be practiced within the scopeof the appended claims. Therefore, the described embodiments should betaken as illustrative and not restrictive, and the invention should notbe limited to the details given herein but should be defined by thefollowing claims and their full scope of equivalents.

What is claimed is:
 1. A method for etching a feature having a featuredepth in a layer of a wafer, the method comprising: etching to a firstdepth of the feature at a first etching rate; etching from the firstdepth to a second depth at a second etching rate, the second etchingrate being slower than the first etching rate; optically determiningwhen the second depth has reached the feature depth; and stoppingetching the feature.
 2. The method of claim 1, wherein etching to afirst depth includes forming a desired profile of the feature.
 3. Themethod of claim 2, wherein etching from the first depth to the seconddepth substantially retains the desired profile of the feature.
 4. Themethod of claim 1, wherein the first etch depth is in the range ofapproximately 65 to 85 percent of the feature depth.
 5. The method ofclaim 1, wherein the feature is a trench.
 6. The method of claim 1,wherein the layer is a crystalline silicon layer.
 7. The method of claim1, wherein the etching depth is optically determined throughout themethod.
 8. The method of claim 7, further comprising opticallydetermining when the first depth has been reached.
 9. The method ofclaim 1, wherein the etching rate is changed by changing the compositionof an etchant.
 10. The method of claim 1, wherein the layer does nothave an associated etch stop indicator.
 11. The method of claim 1,wherein the layer does not have an etch stop layer.
 12. The method ofclaim 1, wherein the optical determination is carried out using anoptical end point technique.
 13. The method of claim 12, wherein theoptical end point technique is selected from the group comprisinginterferometric end point and broadband reflectometry based end point.14. The method of claim 1, wherein the method is part of a processselected from the group comprising recess and shallow trench isolation.15. The method of claim 1, wherein an absolute feature depth isoptically determined.
 16. A method for etching a trench having an endpoint in a silicon layer of a wafer, comprising: etching at a first etchrate; etching at a second etch rate less than the first etch rate;optically determining a current etch depth; and stopping etching so thatthe trench depth reaches the end point.
 17. The method of claim 16,wherein etching at a first etch rate forms a desired trench profile. 18.The method of claim 17, wherein etching at the second etch substantiallyretains the desired etch profile.
 19. The method of claim 16, comprisingoptically determining when to change the etch rate.
 20. Apparatus foretching a feature having a feature depth in a layer of a wafer, theapparatus comprising: an etching tool including a chuck for holding thewafer; an optical end point device adjacent the etching tool andpositioned to measure an etch depth of the feature to be etched; and anelectronic controller in communication with the optical end point deviceand etching tool and configured to reduce the etch rate of the etchingtool at a first etch depth less than the feature depth and stop theetching tool from etching such that the etch depth reaches the featuredepth.